Ambiguous datetime comparisons should use == rather than 'is' for tzinfo comparison

[pganssle]

BPO	28601
Nosy	@tim-one, @abalkin, @pganssle

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/abalkin'
closed_at = None
created_at = <Date 2016-11-03.18:37:19.485>
labels = ['3.7', 'type-bug', 'library']
title = "Ambiguous datetime comparisons should use == rather than 'is' for tzinfo comparison"
updated_at = <Date 2016-11-03.18:46:38.618>
user = 'https://github.com/pganssle'

bugs.python.org fields:

activity = <Date 2016-11-03.18:46:38.618>
actor = 'belopolsky'
assignee = 'belopolsky'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-11-03.18:37:19.485>
creator = 'p-ganssle'
dependencies = []
files = []
hgrepos = []
issue_num = 28601
keywords = []
message_count = 2.0
messages = ['280003', '280005']
nosy_count = 3.0
nosy_names = ['tim.peters', 'belopolsky', 'p-ganssle']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28601'
versions = ['Python 3.6', 'Python 3.7']

[pganssle]

According to PEP-495 (https://www.python.org/dev/peps/pep-0495/#aware-datetime-equality-comparison) datetimes are considered not equal if they are an ambiguous time and have different zones. However, currently "interzone comparison" is defined / implemented as the zones being the same object rather than the zones comparing equal.

One issue with this is that it actually breaks backwards compatibility of the language, because there doesn't seem to be a way to provide a (backwards-compatible) class that implements folding behavior and has equivalent dates compare equal. An example using python-dateutil:

from datetime import datetime
from dateutil import tz

NYC = tz.gettz('America/New_York')
ET = tz.gettz('US/Eastern')

dt = datetime(2011, 11, 6, 5, 30, tzinfo=tz.tzutc()) # This is 2011-11-06 01:30 EDT-4

dt_edt = dt.astimezone(ET)
dt_nyc = dt.astimezone(NYC)

print(dt_nyc == dt_edt)

In Python 3.5 that will return True, in Python 3.6 it will return False, even though 'US/Eastern' and 'America/New_York' are the same zone. In this case, I might be able to enforce that these time zones are singletons so that is always returns True (though this may have other negative consequences for utility), but even that solution would fall apart for things like tzrange and tzstr, where you can know that the dt.utcoffset()s are going to be identical for ALL values of dt, but you can't force the objects to be identical.

I would suggest that it be changed to use __eq__ to determine whether two tzinfo objects are the same zone, as this will allow tzinfo providers to create tzinfo objects with a consistent behavior between versions in this edge case.

[abalkin]

See Datetime-SIG thread <https://mail.python.org/pipermail/datetime-sig/2016-November/001010.html\>.